Earthing of VSD Transformer

[neeb]

We are about to install a number of motors (approximately 600kVA each) each driven by a variable speed drive and associated transfomer. The manufacture is recommending that the transfomer secondary is left unearthed (neutral star point not brought out) i.e. an IT earthing system, since the motor is the only consumer off the transfomer. This sounds a little suspect to me as normal practice would be to always earth the neutral to prevent to the voltage "floating" about.
Anybody any experience or thoughts on this?

 

[DickDV]

Definitely ground the center of the star. The ground fault protection in the inverter will not work properly without it.

And, while inverters generally tolerate power supplies out of balance with respect to ground, there is no need to excessively stress the insulation on the high leg. Ground the center of the star and enjoy nice balanced predictable power.

 

[Skogsgurra]

There is an advantage using an IT system. The capacitive currents from motor winding to ground will be less. That means that the shaft/stator voltage will be less and the resulting bearing EDM will be reduced.

A good compromise is to impedance ground the transformer star point via a reasonably sized resistor - say about 1000 ohms. Or a reactor.

Increasing the impedance for common mode HF signals will also reduce EMI - and that alone can be a reason to have impedance ground, or isolated star point.

Gunnar Englund

http://www.gke.org